Heretofore, a retardation plate includes such as a type pasted on a polarizing plate and a type directly adhered on PVA (a structure combined with a polarizing plate protective film), however, a type in which a polarizing plate protective film has a function of a retardation plate has come to be a main current due to a requirement of decreasing the number of part materials or of cost down of part materials (for example, refer to patent literature 1). These films are subjected to a stretching treatment to be provided with a retardation control function and it has been considered that elastic modulus in the direction perpendicular to the stretching direction should be kept as high as possible to depress the dimension change of a polarizing plate.
For that purpose, it has been necessary to keep a certain amount of orientation remaining in the direction perpendicular to the stretching direction not to make polymer main chains excessively arranged in one direction at the time of stretching. Among retardation film, there is a type in which the film stretching direction and the absorption direction of polarizing film are approximately perpendicular. Heretofore, this film has been considered to be provided with a high elastic modulus as possible in the direction perpendicular to the stretching direction to restrain shrinkage in the absorption direction of a polarizing plate, when it is utilized being pasted on a polarizing plate or as a polarizing plate protective film. However, in the case of elastic modulus being in a range of a general polarizing plate protective film and a retardation plate, such as not more than 5,000 MPa at a film thickness of 80 μm, a further increased elastic modulus scarcely contributes to depression of shrinkage of a polarizing plate and even causes remaining stress in film at stretching, whereby an opposite result against the essential object may result with excess increase of elastic modulus.
Recently, in a liquid crystal TV, front contrast is required to be increased higher than before and various requirements and proposals, such as not only improvement of a polarization degree of a polarizing plate but also control of transparency, moisture permeability and a retardation value of retardation film, and decrease of an anti-glare treatment on the film surface, have been made (for example, refer to patent literatures 2, 3 and 4).
Front contrast is a ratio of a quantity of light at white display to a quantity of light at black display, and rises by increasing a quantity of light at white or decreasing a quantity of light at black. As for retardation film, in addition to the contents described before, proposed has been, for example, a method in which light leak at black display is depressed by restraining haze of film (for example, refer to patent literature 5).
However, contrast is not necessarily raised by the technologies disclosed above and sometimes there may cause phenomena to decrease the contrast.
Further, it has been disclosed that elastic modulus of cellulose ester film in the machine direction and in the direction perpendicular thereto is adjusted into a predetermined range, and, in this range, cellulose ester film, in which values of retardation in the thickness direction and retardation in the plane are suitably adjusted, can be provided. However, only limited information has been indicated concerning front contrast being improved by controlling the ratio of elastic modulus (for example, refer to patent literature 6).
[Patent literature 1] JP-A 2003-270442 (Hereinafter, JP-A refers to Japanese Patent Publication Open to Public Inspection No.)
[Patent literature 1]JP-A 2003-270442 (Hereinafter, JP-A refers toJapanese Patent Publication Open toPublic Inspection No.)[Patent literature 2]JP-A 2005-55601[Patent literature 3]JP-A 2005-17435[Patent literature 4]JP-A 2003-222713[Patent literature 5]JP-A 7-294736[Patent literature 6]JP-A 2001-100039